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          <h1 class="post-title" itemprop="name headline">单例模式的实现方式</h1>
        

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        <p>单例模式一般体现在类声明中，单例的类负责创建自己的对象，同时确保只有单个对象被创建。这个类提供了一种访问其唯一的对象的方式，可以直接访问，不需要实例化该类的对象。</p>
<h3 id="用途"><a href="#用途" class="headerlink" title="用途"></a>用途</h3><p>单例模式有以下两个优点：</p>
<ul>
<li>在内存里只有一个实例，减少了内存的开销，尤其是频繁的创建和销毁实例（比如网站首页页面缓存）。</li>
<li>在选择使用单例模式的时候，不仅仅考虑到其带来的优点，还有可能是有些场景就必须要单例。</li>
</ul>
<h3 id="实现方式"><a href="#实现方式" class="headerlink" title="实现方式"></a>实现方式</h3><h4 id="饿汉模式（线程安全）"><a href="#饿汉模式（线程安全）" class="headerlink" title="饿汉模式（线程安全）"></a>饿汉模式（线程安全）</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Singleton instance = <span class="keyword">new</span> Singleton();  </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span> <span class="params">()</span></span>&#123;&#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getInstance</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> instance;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这种方式基于classloder机制，<code>ClassLoader</code>的<code>loadClass()</code>方法在加载类的时候使用了<code>synchronized</code>关键字，这个方法默认在整个装载过程中都是同步的（线程安全的），实现了lazy loading效果</p>
<h4 id="饿汉模式变种"><a href="#饿汉模式变种" class="headerlink" title="饿汉模式变种"></a>饿汉模式变种</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> Singleton instance = <span class="keyword">null</span>;  </span><br><span class="line">    <span class="keyword">static</span> &#123;  </span><br><span class="line">        instance = <span class="keyword">new</span> Singleton();  </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span> <span class="params">()</span></span>&#123;&#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getInstance</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.instance;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>跟第1种方式差不多，都是在类初始化时就实例化instance。饿汉式单例，在类被加载的时候对象就会实例化。这也许会造成不必要的消耗，因为有可能这个实例根本就不会被用到。而且，如果这个类被多次加载的话也会造成多次实例化。其实解决这个问题的方式有很多，下面提供两种解决方式，第一种是使用静态内部类的形式。第二种是使用懒汉式。</p>
<h4 id="静态内部类"><a href="#静态内部类" class="headerlink" title="静态内部类"></a>静态内部类</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">SingletonHolder</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Singleton INSTANCE = <span class="keyword">new</span> Singleton();  </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span> <span class="params">()</span></span>&#123;&#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">final</span> Singleton <span class="title">getInstance</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> SingletonHolder.INSTANCE;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这种方式同样利用了classloder的机制来保证初始化instance时只有一个线程，<code>Singleton</code>类被装载了，instance不一定被初始化。因为<code>SingletonHolder</code>类没有被主动使用，只有显示通过调用<code>getInstance()</code>方法时，才会显示装载<code>SingletonHolder</code>类，从而实例化instance。达到了延迟加载，并且避免了类被多次加载时创建多个实例的情况。</p>
<h4 id="懒汉模式（线程不安全）"><a href="#懒汉模式（线程不安全）" class="headerlink" title="懒汉模式（线程不安全）"></a>懒汉模式（线程不安全）</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Single</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Single</span><span class="params">()</span></span>&#123;&#125;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Single instance = <span class="keyword">null</span>;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Single <span class="title">getInstance</span><span class="params">()</span></span>&#123;  </span><br><span class="line">        <span class="keyword">if</span>(instance == <span class="keyword">null</span> )  <span class="comment">// 这里会有线程安全问题</span></span><br><span class="line">            instance = <span class="keyword">new</span> Single();</span><br><span class="line">        <span class="keyword">return</span> instance;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="懒汉模式（线程安全）"><a href="#懒汉模式（线程安全）" class="headerlink" title="懒汉模式（线程安全）"></a>懒汉模式（线程安全）</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Singleton instance;  </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span> <span class="params">()</span></span>&#123;&#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">synchronized</span> Singleton <span class="title">getInstance</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">if</span> (instance == <span class="keyword">null</span>) &#123;  </span><br><span class="line">            instance = <span class="keyword">new</span> Singleton();  </span><br><span class="line">        &#125;  </span><br><span class="line">        <span class="keyword">return</span> instance;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这种写法能够在多线程中很好的工作，而且看起来它也具备很好的lazy loading，但是，效率很低，99%情况下不需要同步。解决这个问题可以通过双重校验锁来解决。</p>
<h4 id="使用final"><a href="#使用final" class="headerlink" title="使用final"></a>使用final</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">FinalWrapper</span>&lt;<span class="title">T</span>&gt; </span>&#123;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">final</span> T value;</span><br><span class="line">    </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">FinalWrapper</span><span class="params">(T value)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.value = value;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">FinalSingleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> FinalWrapper&lt;FinalSingleton&gt; helperWrapper = <span class="keyword">null</span>;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> FinalSingleton <span class="title">getHelper</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        FinalWrapper&lt;FinalSingleton&gt; wrapper = helperWrapper;</span><br><span class="line">        <span class="keyword">if</span> (wrapper == <span class="keyword">null</span>) &#123;</span><br><span class="line">            <span class="keyword">synchronized</span> (<span class="keyword">this</span>) &#123;</span><br><span class="line">                <span class="keyword">if</span> (helperWrapper == <span class="keyword">null</span>) &#123;</span><br><span class="line">                    helperWrapper = <span class="keyword">new</span> FinalWrapper&lt;FinalSingleton&gt;(<span class="keyword">new</span> FinalSingleton());</span><br><span class="line">                &#125;</span><br><span class="line">                wrapper = helperWrapper;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> wrapper.value;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="CAS实现-通过Java原子变量的操作类AtomicReference实现"><a href="#CAS实现-通过Java原子变量的操作类AtomicReference实现" class="headerlink" title="CAS实现(通过Java原子变量的操作类AtomicReference实现)"></a>CAS实现(通过Java原子变量的操作类AtomicReference实现)</h4><p>关于<code>AtomicReference</code>和CAS，我的其他文章有简单的介绍：<a href="/blog/Multithreading_atomic_cas.html" title="Java中的Atomic以及CAS原理">Java中的Atomic以及CAS原理</a></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> AtomicReference&lt;Singleton&gt; INSTANCE = <span class="keyword">new</span> AtomicReference&lt;Singleton&gt;(); </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span><span class="params">()</span> </span>&#123;&#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getInstance</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">for</span> (;;) &#123;</span><br><span class="line">            Singleton singleton = INSTANCE.get();</span><br><span class="line">            <span class="keyword">if</span> (<span class="keyword">null</span> != singleton) &#123;</span><br><span class="line">                <span class="keyword">return</span> singleton;</span><br><span class="line">            &#125;</span><br><span class="line">            singleton = <span class="keyword">new</span> Singleton();</span><br><span class="line">            <span class="keyword">if</span> (INSTANCE.compareAndSet(<span class="keyword">null</span>, singleton)) &#123;</span><br><span class="line">                <span class="keyword">return</span> singleton;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="枚举"><a href="#枚举" class="headerlink" title="枚举"></a>枚举</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">enum</span> Singleton &#123;  </span><br><span class="line">    INSTANCE;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>使用枚举就不需要额外代码来保证线程安全，因为java在底层已经做了线程安全方面的保证的。<br>定义枚举时使用enum和class一样，是Java中的一个关键字。就像class对应用一个Class类一样，enum也对应有一个Enum类。通过将定义好的枚举反编译，我们就能发现，其实枚举在经过javac的编译之后，会被转换成形如public final class T extends Enum的定义。而且，枚举中的各个枚举项同事通过static来定义的。如：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">enum</span> T &#123;</span><br><span class="line">    SPRING;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>反编译后代码为：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">final</span> <span class="class"><span class="keyword">class</span> <span class="title">T</span> <span class="keyword">extends</span> <span class="title">Enum</span> </span>&#123;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">final</span> T SPRING;</span><br><span class="line">    <span class="keyword">static</span> &#123;</span><br><span class="line">        SPRING = <span class="keyword">new</span> T(<span class="string">"SPRING"</span>, <span class="number">0</span>);</span><br><span class="line">        ENUM$VALUES = (<span class="keyword">new</span> T[]&#123;</span><br><span class="line">            SPRING     </span><br><span class="line">        &#125;);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>根据JVM的类加载机制可知，static类型的属性会在类被加载之后被初始化，<strong>当一个Java类第一次被真正使用到的时候静态资源被初始化、Java<br>类的加载和初始化过程都是线程安全的（因为虚拟机在加载枚举的类的时候，会使用ClassLoader的loadClass方法，而这个方法使用同步代码块保证了线程安全）</strong>，所以，创建一个enum类型是线程安全的。</p>
<h4 id="双重校验锁"><a href="#双重校验锁" class="headerlink" title="双重校验锁"></a>双重校验锁</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;  </span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">volatile</span> <span class="keyword">static</span> Singleton singleton;  </span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Singleton</span> <span class="params">()</span></span>&#123;&#125;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Singleton <span class="title">getSingleton</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">if</span> (singleton == <span class="keyword">null</span>) &#123;  </span><br><span class="line">            <span class="keyword">synchronized</span> (Singleton.class) &#123;  </span><br><span class="line">                <span class="keyword">if</span> (singleton == <span class="keyword">null</span>) &#123;  </span><br><span class="line">                    singleton = <span class="keyword">new</span> Singleton();  </span><br><span class="line">                &#125;  </span><br><span class="line">            &#125;  </span><br><span class="line">        &#125;  </span><br><span class="line">        <span class="keyword">return</span> singleton;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这里必须使用<code>volatile</code>关键字来修饰这个单例对象，因为如果不使用<code>volatile</code>，这段代码将是不可靠的，原因如下：</p>
<ul>
<li>编译器优化了程序指令, 以加快cpu处理速度</li>
<li>多核cpu动态调整指令顺序, 以加快并行运算能力</li>
</ul>
<p>问题出现的顺序:</p>
<ol>
<li>线程A, 发现对象未实例化, 准备开始实例化</li>
<li>由于编译器优化了程序指令, 允许对象在构造函数未调用完前, 将 共享变量的引用指向 部分构造的对象, 虽然对象未完全实例化, 但已经不为null了</li>
<li>线程B, 发现部分构造的对象已不是null, 则直接返回了该对象</li>
</ol>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>上面的单例实现方式，除了枚举方式外，其他方式都有一个共同的弊端，那就是单例有可能被序列化锁破坏。普通的Java类的反序列化过程中，会通过反射调用类的默认构造函数来初始化对象。所以，即使单例中构造函数是私有的，也会被反射给破坏掉。由于反序列化后的对象是重新new出来的，所以这就破坏了单例。<br>而enum类型在序列化的时候Java仅仅是将枚举对象的name属性输出到结果中，反序列化并不是通过反射实现的，而是通过java.lang.Enum的valueOf<br>方法来根据名字查找枚举对象。同时，编译器是不允许任何对这种序列化机制的定制的，因此禁用了writeObject、readObject等方法。</p>
<h3 id="JDK中的单例模式"><a href="#JDK中的单例模式" class="headerlink" title="JDK中的单例模式"></a>JDK中的单例模式</h3><p><code>java.lang.Runtime</code>类封装了Java运行时的环境。每一个java程序实际上都是启动了一个JVM进程，那么每个JVM进程都是对应这一个<code>Runtime</code>实例，此实例是由JVM为其实例化的。每个 Java<br>应用程序都有一个 <code>Runtime</code> 类实例，使应用程序能够与其运行的环境相连接。<br>由于Java是单进程的，所以，在一个JVM中，<code>Runtime</code>的实例应该只有一个，所以应该使用单例来实现。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Runtime</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Runtime currentRuntime = <span class="keyword">new</span> Runtime();</span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Runtime <span class="title">getRuntime</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> currentRuntime;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="title">Runtime</span><span class="params">()</span> </span>&#123;&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这其实是饿汉式单例模式。在该类第一次被classloader加载的时候，这个实例就被创建出来了。</p>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#用途"><span class="nav-number">1.</span> <span class="nav-text">用途</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#实现方式"><span class="nav-number">2.</span> <span class="nav-text">实现方式</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#饿汉模式（线程安全）"><span class="nav-number">2.1.</span> <span class="nav-text">饿汉模式（线程安全）</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#饿汉模式变种"><span class="nav-number">2.2.</span> <span class="nav-text">饿汉模式变种</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#静态内部类"><span class="nav-number">2.3.</span> <span class="nav-text">静态内部类</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#懒汉模式（线程不安全）"><span class="nav-number">2.4.</span> <span class="nav-text">懒汉模式（线程不安全）</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#懒汉模式（线程安全）"><span class="nav-number">2.5.</span> <span class="nav-text">懒汉模式（线程安全）</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#使用final"><span class="nav-number">2.6.</span> <span class="nav-text">使用final</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#CAS实现-通过Java原子变量的操作类AtomicReference实现"><span class="nav-number">2.7.</span> <span class="nav-text">CAS实现(通过Java原子变量的操作类AtomicReference实现)</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#枚举"><span class="nav-number">2.8.</span> <span class="nav-text">枚举</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#双重校验锁"><span class="nav-number">2.9.</span> <span class="nav-text">双重校验锁</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#总结"><span class="nav-number">3.</span> <span class="nav-text">总结</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#JDK中的单例模式"><span class="nav-number">4.</span> <span class="nav-text">JDK中的单例模式</span></a></li></ol></div>
            

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  <script type="text/javascript">
    // Popup Window;
    var isfetched = false;
    var isXml = true;
    // Search DB path;
    var search_path = "search.xml";
    if (search_path.length === 0) {
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    // monitor main search box;

    var onPopupClose = function (e) {
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      $('#local-search-input').val('');
      $('.search-result-list').remove();
      $('#no-result').remove();
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      $('body').css('overflow', '');
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    function proceedsearch() {
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay"></div>')
        .css('overflow', 'hidden');
      $('.search-popup-overlay').click(onPopupClose);
      $('.popup').toggle();
      var $localSearchInput = $('#local-search-input');
      $localSearchInput.attr("autocapitalize", "none");
      $localSearchInput.attr("autocorrect", "off");
      $localSearchInput.focus();
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    // search function;
    var searchFunc = function(path, search_id, content_id) {
      'use strict';

      // start loading animation
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay">' +
          '<div id="search-loading-icon">' +
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          '</div>' +
          '</div>')
        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

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          var resultContent = document.getElementById(content_id);
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                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
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                  // merge hits into slices

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                    var position = item.position;
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                      // move to next position of hit

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                      while (index.length != 0) {
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                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
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                        } else {
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                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
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                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
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                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
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                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
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                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
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                  // highlight title and content

                  function highlightKeyword(text, slice) {
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                      var end = hit.position + hit.length;
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                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
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            } else {
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                } else if (resultLeft.hitCount !== resultRight.hitCount) {
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                } else {
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                }
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              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
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              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
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          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
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                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
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